Preparation of metal powders



Patented Aug. 31, 1954 UNITED STATES TENT OFFICE Thomas P. Whaley, Detroit, Mich., assignor to Ethyl Corporation, New York, N. Y., a corporation of Delaware No Drawing. Application April 22, 1952, Serial No. 283,750

13 Claims. 1

The present invention relates to metal powders and more particularly to a process for preparing finely divided metal powders from metal compounds.

Metal powders for use in powder metallurgy have long been prepared by processes involving mechanical subdivision of the metal such as grinding or milling. Such processes are still used to a great extent and are generally satisfactory for producing metal powders for conventional applications. This technique, however, is subject to the disadvantage in that a wide range of particle sizes is obtained, and hence where uniformity of size is required for a particular application considerable screening and classification is necessary. These operations add expense to the process itself, and also increase the cost of the finished product. Another disadvantage is that extremely fine metal powders cannot be conveniently prepared by mere mechanical subdivision. Because of these disadvantages, chemical means have been sought for preparing finely divided metal powders. One such process which is used extensively consists of the pyrolysis of metal compounds such as carbonyls and halides. While such processes generally produce good quality powders the high temperatures required have some adverse effects on the products. High temperatures, for example, tend to cause some degree of agglomeration of the particles so that the resulting powder is not as fine as would be the case if lower temperatures could be employed. It is also believed that in preparing iron powder for permanent magnets a high temperature will tend to adversely afiect the magnetic properties of the final product.

It is therefore an object of this invention to overcome the disadvantages of the prior art and provide a novel and improved method for preparing metal powders. A further object of this invention is to provide an improved low temperature process for preparing metal powders from the respective metal compounds. Another object is to provide a process for producing finely divided iron powder by a low temperature process so as to render the product well suited for use in the fabrication of permanent magnets. A still further object of this invention is to provide a process for conveniently preparing powdered metal alloys. These and other objects of the present invention will become apparent as the description proceeds and the features of novelty which characterize the invention are pointed out with particularity in the 2 claims annexed to and forming a part of this specification.

The above and other objects of the present invention are accomplished by reducing a metal compound with alkali metal in the form of subdivided particles dispersed or suspended in an inert medium. It has long been known that the alkali metals, and particularly sodium, can be employed as reducing agents to win metals from their respective ores and compounds. These processes, however, generally employed high temperatures and the resulting metal product was not in the form of finely subdivided particles. I have discovered, however, that extremely fine and in most cases pyrophoric metal powders can be prepared by reducing metal compounds with alkali metal which is in the unique form. of subdivided particles suspended or dispersed in an inert liquid medium. My process is particularly advantageous in that temperatures substantially below the melting point of the metal being produced are used. Because of these low operating temperatures the process is particularly well suited to the preparation of iron powder to be used in the manufacture of permanent magnets. Substantially quantitative yields of powder are obtained, and the product is characterized by the uniformity and high degree of fineness of the particles. In most cases the particles areso fine as to be pyrophoric, thus indicating particle sizes of the order of 10 microns or less.

The alkali metal which functions as a reducing agent in accordance with my invention is employed in the form of subdivided particles dispersed or suspended in an inert liquid medium. Alkali metal dispersions of this nature are well known in the art, and are generally prepared by fusing a mass of alkali metal in an inert liquid and agitating the entire mixture while maintaining the temperature above the melting point of the alkali metal. The agitation can be conveniently accomplished by means of a counter-rotating stirrer operated at speeds from about 8,000 to 15,000 R. P. M. although it is ob- 4 an alkali metal concentration from about 5 to 50 per cent by weight.

A dispersion prepared by the foregoing process consists of finely divided particles of alkali metal suspended in and uniformly dispersed throughout the liquid medium. The alkali metal particle size depends to some extent upon the degree of agitation and generally varies between about 1 and 50 microns. For use in the present invention it is preferred that the dispersion have an average particle size of between about 1 and 20 microns.

In preparing an alkali metal dispersion a wide choice of dispersing mediums is available. In general, organic liquids inert to alkali metal and having a boiling point above the melting point of the particular alkali metal employed are preferred. Typical examples of inert mediums which can be used are benzene, xylene, toluene, gasoline, kerosene, mineral oil, heavy alkylate, noctane, n-decane, and the like. Still other liquids can be employed as dispersing mediums, the foregoing list merely representing those most commonly used.

Of the various alkali metals any one or more can be employed as reducing agents in my process although sodium is preferred. Potassium, lithium, cesium and any other alkali metal can be used as well as mixtures of two or more alkali metals with equally good results.

The following example will serve to illustrate the preparation of an alkali metal dispersion of the type described above. In this example the parts and proportions given are on a Weight basis.

Example I Fifty parts of metallic sodium are fused in a vessel containing 120 parts of toluene. The vessel is equipped with a high-speed counter-rotating stirrer, and the mixture is agitated with the stirrer operating at about 12,000 E. P. M. for about 30 minutes. During the agitation the temperature of the mixture is maintained at about 110 C. so as to insure that the sodium is at all times in the molten state. The resulting dispersion has an average metal particle size of about microns.

When preparing alkali metal dispersions it is frequently advantageous to incorporate therein a small amount of a dispersing agent which functions to enhance the formation of uniformly sized particles and to prevent settling and agglomeration of these particles once the dispersion is formed. The dispersing agent can be added during the agitation step or, if desired, it can be introduced into the initial mixture before agitation is begun. Examples of typical dispersing agents most commonly used are fatty acids, activated carbon, charcoal, carbon black and the like. The amount of dispersing agent used is small and generally does not exceed about 2 per cent by weight of the total mixture.

In accordance with my invention metal powders are prepared by reducing a metal compound with alkali metal in the form of particles dispersed in an inert liquid. Although the invention can be successfully applied to any metal compound capable of being reduced by an alkali metal, the halogen salts of the various metals are preferred. Thus, the corresponding metal powders can be prepared from metal halides such as, for example, ferric chloride, ferrous chloride, ferric fluoride, zinc chloride, cupric chloride, silver chloride, chromic chloride, cobaltous chloride,

cobaltous bromide, cadmium bromide, stannic chloride, stannous chloride, stannic bromide, stannic iodide, manganese dichloride, nickel chloride, nickel iodide, molybdenum pentachloride, silver bromide and other metal halides. Although the halides are the preferred starting materials other inorganic as well as organic metal compounds can be employed such as, for example, the sulfates, carbonates, acetates, formates, and the like of the above and other metals. Specific examples of such other compounds include nickel sulfate, nickel acetate, nickel formate, chromic sulfate, cobaltous acetate, ferric sulfate, and the like. In general, however, the oxides are somewhat dimcult to reduce and for this reason they are not preferred since they require elevated temperatures for reduction.

The reduction is carried out by commingling the metal compound and the alkali metal dispersion in any suitable reaction vessel. The reactants can be combined by adding the metal compound to a vessel containing the dispersion or, as an alternative, the metal compound can be placed in a vessel containing an inert medium, such as benzene or toluene, and the alkali metal dispersion added to this mixture. Still another method of carrying out the reduction comprises dissolving the metal compound in a suitable medium and thereafter commingling the dispersion and this solution. When the reactants are brought together by any of the foregoing methods reduction of the metal ,compound takes place readily. Since the reaction itself is usually exothermic means can be provided for chilling the reaction vessel when it is desired to maintain a relatively low temperature as in the preparation of iron powder. The finely divided metal powder which is formed can be recovered from the reaction mixture by conventional means such as filtration or centrifugation, although the extreme fineness of the product renders the latter more suitable. Prior to recovering the powder from the reaction mixture any excess unreacted alkali metal can be killed by the addition of an alcohol such as methanol or isopropyl alcohol so as to form alcoholate. After treating the reaction mixtures with alcohol, water from which dissolved oxygen has been dispelled is added so as to dissolve the alcoholate, alkali metal salts, and any other impurities that may be present. The metal powder can then be recovered readily from the reaction mixture.

One of the outstanding and novel features of my process is that relatively low temperatures are utilized. For example, when preparing iron powder from ferric chloride the reaction with dispersed sodium occurs readily at room temperature and yields an iron powder which is so finely divided as to be pyrophoric. When reducing other metal compounds such as cupric chloride, zinc fluoride, chromic chloride and the like it is frequently necessary to apply some heat to initiate the reaction. It is generally, however, not necessary to heat the reaction mixture to above the melting point of the alkali metal, and it is preferred that the reaction be conducted at temperatures below the melting point of the alkali metal. When preparing iron powders to be used in making permanent magnets it is preferred that the reaction temperature be maintained below about 40 C.

The powders prepared in accordance with this invention are so finely subdivided as to be pyrophoric when exposed to air. In order to prevent such rapid oxidation of the product the reaction Example II Five parts of anhydrous ferric chloride was added to about 50 parts benzene contained in a reaction vessel. The vessel was placed under an inert atmosphere of dry nitrogen and parts of a 42 per cent dispersion of sodium in toluene was added. The reaction mixture was agitated by means of a mechanical stirrer and the temperature was maintained at about 30 C. by externally chilling the reaction vessel. Reaction occurred readily resulting in the reduction of the chloride to form small particles of free iron. The end of the reaction was noted by the absence of heat evolution, and thereupon the excess sodium was killed by the addition of isopropyl alcohol. After the alcohol-sodium reaction had ceased, 500 parts of water were added for the purpose of dissolving alcoholate, sodium chloride and any other impurities. Iron powder settled to the bottom of the vessel in the form of a black precipitate and was removed from the reaction mixture by means of a centrifuge.

Example III Three parts of sodium dispersed in 3 parts toluene was placed in a reaction vessel containing about 56 parts toluene and provided with a stirrer and means for chilling the vessel. The vessel was maintained in an inert atmosphere and ferric chloride was slowly added while agitating thereactants by means of the stirrer. Upon the addition of ferric chloride an exothermic reaction occurred, and in order to maintain a temperature of between -30 C. it was necessary to chill the vessel. After a period of one hour an equivalent amount of ferric chloride had been added and continued addition gave no further reaction. The reaction mass was treated with alcohol and then water as described in the preceding example, and the iron powder which had formed during the reaction was separated by centrifugation. The separated powder was washed with alcohol, acetone and finally dried under vacuum. The product was iron powder in the form of finely divided black particles which oxidized readily upon exposure to air.

Example IV A solution of 5.7 parts ferric chloride in 124.1 parts diethyl ether was initially prepared and then slowly added to 4 parts of sodium dispersed in 50 parts heavy alkylate contained in a reaction vessel. The vessel was provided with a cooling means and stirrer, and the temperature was maintained at between 10 C. and 20 C. throughout the addition of the ferric chloride solution. The entire amount of chloride solution was added during a period of 30 minutes, and the reaction persisted for about 20 minutes after the addition was complete. Any unreacted sodium was then killed by adding 12 parts isopropyl alcohol, and thereafter the reaction mixture was treated with water so as to dissolve the alcoholate, sodium chloride and any other impurities. Finely divided black iron powder which was observed to be suspended in the reaction mixture was recovered therefrom by centrifugation, washed with alcohol and acetone, and dried under vacuum.

Example V Copper powder was prepared following the procedure described in Example II using approximately 2 parts of sodium dispersed in 2 parts alkylate and 1 part cupric chloride. The dispersion was placed in a reaction vessel containing 25 parts alkylate and the cupric chloride was then added. The reaction vessel was heated to between and 90 C., and the chloride was readily reduced yielding finely divided copper powder.

Example VI Zinc powder was prepared from zinc fluoride following the procedure described in Example II using 3 parts of a 50 per cent dispersion of sodium in alkylate and 1 part zinc fluoride. A temperature of C. was maintained during the reaction and the fluoride was reduced yielding finely divided free zinc.

Example VII Chromic chloride was used to prepare chromium powder by the process as described in Example II using 1 part of chromic chloride and 4 parts of a 50 per cent dispersion of sodium in alkylate. At a temperature of 85 C. the chloride was reduced to yield a finely divided chromium powder.

When other metal compounds which are reducible by an alkali metal are substituted for the metal halides of the above examples equally good results are obtained. For example, powdered cobalt can be prepared from cobalt compounds such as cobaltous chloride and cobaltous acetate; iron from ferric sulfate and iron formate; tin from stannic chloride; cadmium from cadmium bromide; silver from silver iodide and silver bro mide; molybdenum from molybdenum pentachloride, and the like. Similarly, other alkali metals can be employed. For example, dispersions of potassium, lithium, cesium or dispersions of two or more alkali metals, such as a dispersion of sodium and potassium when substituted in the above examples for the sodium dispersion produce equally good results.

My process is also useful in the preparation of alloy powders. In preparing alloy powders the procedure outlined in the above examples is followed except that compounds of two or more different metals are reduced simultaneously with the dispersed alkali metal. For example, a copper-zinc alloy powder is prepared by simultaneously reducing a mixture of cupric chloride and zinc fluoride by substituting this mixture for the metal halides. in the above examples. Alloy powders of iron and chromium, silver and copper, copper and tin, tin and zinc, as well as other mixtures of different metal powders are prepared in like manner.

The powders prepared in accordance with my invention can be used in conventional powder metallurgy practices wherein fabricated articles are made by pressure molding techniques. Iron powder prepared by my process is particularly useful in the manufacture of permanent magnets.

It is to be understood that the above examples are given only by way of illustration and I intend by the appended claims to cover all modifications which fall Within the spirit and scope of my invention.

I claim:

1. A process for the preparation of metal powders comprising reducing at least one metal com- 7 pound other than an alkali metal compound with an alkali metal, said alkali metal being in the form of finely divided particles suspended in an inert organic liquid.

2. A process for preparing iron powder comprising reducing ferric chloride with an alkali metal, said alkali metal being in the form of finely divided particles suspended in an inert organic liquid.

3. The process as defined in claim 2 wherein said alkali metal is sodium.

4. A process for preparing iron powder comprising reducing an iron compound with alkali metal in the form of finely divided particles suspended in an inert organic liquid and recovering said iron powder from the reaction mixture.

5. In a process for the preparation of metal powders by reducing metal salts with alkali metal, the improvement comprising reacting said alkali metal in the form. of finely divided particles suspended in an inert organic liquid with said metal salt, and separating said metal powder from the reaction mixture.

6. Ln a process for the preparation of metal powder by reducing metal halide with an alkali metal, the improvement comprising introducing said metal halide into a suspension of finely divided particles of alkali metal in an inert organic liquid, and separating said metal powder from the reaction mixture.

7. A process for preparing copper powder comprising reducing cupric chloride with sodium metal, said metal being in the form of finely divided particles suspended in an inert organic liquid.

8. A process for preparing zinc powder comprising reducing zinc chloride with sodium metal, said sodium metal being in the form of finely divided particles suspended in an inert organic liquid.

9; A process for preparing chromium powder comprising reducing chromic chloride with sodium metal, said sodium metal being in the form or finely divided particles suspended in an inert organic liquid.

10'. A process for preparing alloy powders comprising reducing simultaneously a mixture of two or more different metal compounds other than an alkali metal compound with an alkali metal, said alkali metal being in the form of finely divided particles suspended in an inert organic liquid.

11. A process for the preparation of a copperzinc alloy powder comprising reducing a mixture of cupric chloride and zinc fluoride with sodium metal, said sodium metal being in the form of finely divided particles suspended in an inert organic liquid.

12. The process of claim 6 wherein the reduction is efiected at a temperature below the melting point of the alkali metal.

13. A process for preparing iron powder comprising reducing ferric chloride with sodium metal, said sodium metal being in the form of finely divided particles suspended in an organic liquid inert to said sodium metal and having a boiling point above the melting point of said sodium metal at a concentration not greater than 60% by weight and having an average particle size not greater than 50 microns, said reduction being conducted at a temperature below 40 C., and recovering said iron powder from the reaction mixture.

References Cited in the file of this patent UNITED STATES PATENTS Number 

1. A PROCESS FOR THE PREPARATION OF METAL POWDERS COMPRISING REDUCING AT LEAST ONE METAL COMPOUND OTHER THAN AN ALKALI METAL COMPOUND WITH AN ALKALI METAL, SAID ALKALI METAL BEING IN THE FORM OF FINELY DIVIDED PARTICLES SUSPENDED IN AN INERT ORGANIC LIQUID. 